Orthogonal Frequency-Division Multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels (sub-carriers) at different frequencies.
The transmit power for WLAN or WPAN devices in a network needs to be precisely controlled. It is beneficial to maximize the transmit power level to enhance the range and the quality of transmission. On the other hand, it is mandatory to stay below the maximum transmit peak power allowed by the spectrum regulation. However, dispersion of integrated circuit characteristics, as well as variations mainly due to operating temperatures cause a lack of precision in the transmit power level.
It is thus highly desirable to constantly calibrate the peak transmit power to its optimum level without adding complexity to the WLAN device. One difficulty comes from the fact that in an OFDM based transmission, the frequency band of the transmit signal is generally large or even very large (e.g., hundreds of megahertz). This results in variations of the signal level across the band. Another difficulty is in precisely measuring the transmitted energy across the whole frequency band.
One additional difficulty is that, at any time, no spurious emission is authorized by the Regulation. Thus, measurement of the transmit power needs to be done without radiating energy externally to the tested device.